If you could only do one, which one and why? Travel time is June.
I've been to both, and while each was nice enough, neither was a "wow." That said, Toulouse is larger, and had more to see and do. I saw Albi (in particular, the Toulouse Lautrec museum) as a daytrip from Toulouse. From Montpellier, you could see Nime as a daytrip (I saw it from Arles on another trip, and loved it).